The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
The invention relates to a process of melting ferrous metal using a liquid or a gaseous fuel or pulverized solid fuel in a reverberatory furnace consisting of a hearth, an sloped melting chamber and a recuperator whereas a burner system is installed on the hearth for combustion of the fuel and oxygen bearing gas, the hearth under the burner acts as a superheater to achieve the temperature necessary for alloying and to receive the molten metal from the sloped melting chamber, the sloped melting chamber is charged from one end by moving gases products of combustion and to which is fed solid metals in a batch or continuous mode and from which the waste gases are redirected to a recuperator or a regenerator to pre-heat the oxygen bearing gases necessary for combustion, and whereas the hearth has the necessary discharge openings to remove molten metal and slag.
The melting of ferrous alloys has been done traditionally, using coke in the cupola, open-hearth and reverberatory furnaces leading to high generation of carbon monoxide and needing expensive cleaning of flue gases, or through the use of expensive electricity which is not always available in developing country. Natural gas and oil production is anticipated to increase in the years to come from multiple sources leading to new opportunities for the foundry industry by using these fuels. Natural gas particularly, and certain fuel derived gases offer a cleaner process than the coke cupola for melting iron and has been applied to the development of cokeless cupolas (U.S. Pat. No. 3,803,678) leading to a much reduced level of carbon monoxide and sulfur emissions.
Conceptually the cokeless cupola was developed to use natural gas for melting iron. The low eutectic temperature has lead to the addition of an electric super-heater, typically an induction channel furnace, on the discharge of the cupola to reach higher temperature and add graphite. This is the basis of the TAFT process (U.S. Pat. No. 3,802,678). However the capital investment in a duplex system for a TAFT cokeless cupola followed by an induction furnace has limited the use of the process in foundries.
In order to carry the charges load in a vertical shaft the TAFT cokeless cupola (U.S. Pat. No. 3,802,678). Features a water cooled grate supporting ceramic balls as a bed under the charge. The flame temperature must therefore be cooled as it passes through the grate.
In order to eliminate the use of a duplex system of a gas-fired cupola, a recuperator can be installed on the flue gases of a reverberatory—cupola furnace (U.S. Pat. No. 4,758,270)—This is also done on oil-fired reverberatory-cupola Wuest Oil furnace shown on page 156 to 158 of J. E. Hurst—“Melting Iron in the Cupola—Penton Publishing Co, 1929, Reprinted by Lindsay Publications Inc ISBN 1-55918-102-8—The author as early as 1929 claimed that the use of the recuperator would eliminate the need for duplex the cupola with an electric furnace.
In U.S. Pat. No. 4,758,270 it is proposed that the temperature of the oxygen-containing gas (air) can reach 400 to 600° C. (752 to 1112° F.) so that the flame temperature can reach at least 1800° C. (3272° F.) which is sufficient to melt iron at the temperature of 1450° C. (2642° F.)
The arrangements of a combined reverberatory-cupola furnace such as the Wuest Oil furnace uses the vertical shaft or cupola section as a charging area. It must therefore be tall or of large diameter to accept a large charge. The tall cupola cools the gases before they enter the recuperator. The said recuperator being of refractory or tube construction. For tube construction the temperature of the flue gases must be lowered to 700 degrees Celsius or 1292 degrees Fahrenheit to avoid melting the recuperator, and using special tubes in Nickel-Chromium Austenitic Steels
For cokeless furnaces, whether gas fired or oil fired, carbon in a graphite form is added through a special opening under the burners.
In inventions covered by patents
U.S. Pat. No. 3,578,302
U.S. Pat. No. 3,803,678
U.S. Pat. No. 3,948,647
U.S. Pat. No. 4,877,449
The vertical shaft used to feed and charge the solid load of metal is supported by water cooled grid. This complicates the design of the cupola, and can cause a risk of explosion if the lining of the grid starts to melt away. As the flame passes through the grid it must be cooled. U.S. Pat. No. 4,758,270 tries to surmount this problem through the use of a reverberatory hearth to which the burner is connected.
U.S. Pat. No. 2,734,818 also shows a direct fired reverberatory furnace for melting stainless steel without a cooling grid. This invention requires however that the furnace be pre-heated to 3000 F. prior to charging the solid metal. However as this design does not use a recuperator, oxygen lance is used instead to achieve the required high temperatures. U.S. Pat. No. 2,734,818 also shows a method of tilting the furnace to empty it.
U.S. Pat. No. 2,755,180 also shows a direct fired reverberatory furnace for melting iron without a cooling grid. This invention requires however proper lining of the furnace with alumina-silica bricks and the use of calcium oxide to control erosion of the lining.
U.S. Pat. No. 924,025 designed for a gas fired or liquid fired cupola, a water cooled grid is not used, but the entire cupola can be inclined from a vertical position to an inclined position as needed.
The Wuest Oil Reverberatory furnace did not use a water cooled grid to support the charge, but the hearth featured sloped walls to facilitate the flow of molten metal.
U.S. Pat. No. 4,369,955 proposes that the exhaust gases from a cupola be diverted to an inclined furnace feeding the cupola, and that the charge of gray iron needing melting, compressed air and fuel are introduced with the said furnace being preferably of a reverberatory roof design
U.S. Pat. No. 4,935,489 proposes that the natural gas be burned just above the stoichiometric ratio through the burner below the scrap iron, sponge or solid iron, and that further fresh air and oxygen be added above the charge to complete the conversion of excessive carbon monoxide into carbon dioxide and reduce emissions. The vertical shaft is effectively used as an afterburner.
U.S. Pat. No. 4,758,270 proposes that the natural gas be burned just above the stoichiometric ratio through the burner below the scrap iron, sponge or solid iron, and that further fresh oxygen be added above the charge to complete the conversion of excessive carbon monoxide into carbon dioxide and reduce emissions. The vertical shaft is effectively used as an afterburner.